1. Field of the Invention
The present relates, generally, to a method and communication system for transmitting information with the aid of a multicarrier method and, more specifically, to such a method and system wherein maximum utilization of available transmission resources of a transmission medium is achieved during the transmission of information via the transmission medium which has frequency-selective transmission characteristics.
2. Description of the Prior Art
In wireless communication networks based on radio channels, especially in point-to-multipoint radio feeder networks (also called “radio in the local loop” or, respectively, “RLL”), a number of network terminating units are, in each case, connected to a base station (also called “radio base station” or, respectively “RBS”) via one or more radio channels. In telcom report No. 18 (1995), vol. 1 “Drahtlos zum Freizeichen” [Wireless to the ringing tone] page 36, 37, for example, a wireless feeder network for the wireless speech and data communication is described. The communication system described represents an RLL subscriber line in combination with a modern broadband infrastructure, e.g. “fiber to the curb”, which can be implemented within a short time and without great expenditure instead of running wire-connected local loops. The network terminating units RNT allocated to the individual subscribers are connected to a higher-level communication network, for example to the ISDN-oriented landline network, via the “radio channel” transmission medium and the base station RBS.
Due to the increasing spread of multimedia applications, high-bit-rate data streams must be transmitted rapidly and reliably via communication networks, especially via wireless communication networks or, respectively, via mobile radio systems, and high demands are made on the radio transmission systems which are based on a transmission medium “radio channel” which is susceptible to interference and difficult to assess with regard to the quality of transmission. A transmission method for transmitting broadband data streams, such as video data streams, is represented by, for example, the OFDM (orthogonal frequency division multiplexing) transmission method based on a so-called multicarrier method. In the OFDM transmission technology, the information to be transferred or, respectively, the data stream to be transferred is divided or, respectively, converted to parallel form, to a number of sub-channels or subcarriers within the radio channel. The information to be transferred in each case being transmitted at a relatively low data rate but in parallel in relatively superimposed form. The OFDM transmission technology is used, for example, in digital terrestrial radio (also called digital audio broadcasting DAB) and for digital terrestrial television (also called digital terrestrial video broadcasting DTVB).
The OFDM transmission method is described in greater detail in the printed document “Mitteilungen der TU-Braunschweig, Mobilfunktechnik für Multimedia-Anwendungen” (Information as the Braunschweig technical university, mobile radio technology for multimedia applications), Professor H. Rohling, volume XXXI, issue 1-1996, in figure 6, page 46. In this method, a serial/parallel conversion is performed for the modulation of, for example, the n subcarriers on the basis of a serial data stream in the transmitter, a binary code word with word length k (the word length k being dependent on the modulation method used) being formed in each case for the ith OFDM block in time with block length T′ and the jth subcarrier. From the code words formed, the corresponding complex modulation symbols, also called transmit symbols in the text which follows, are formed with the aid of a transmitter-specific modulation method, wherein one transmit symbol is allocated to each of the k subcarriers at any time i. The spacing of the individual subcarriers is defined by Δf=1−T′ which guarantees that the individual subcarrier signals are orthogonal within the useful interval [0, T′]. By multiplying the oscillations of the individual subcarriers by the corresponding modulation symbols or transmit symbols and subsequently adding the modulation products formed, the corresponding discrete-time transmit signal is generated for the ith OFDM block in time. This transmit signal is calculated in sampled, i.e. in discrete-time form by an inverse discrete Fourier transform (IDFT) directly from the modulation symbols or transmit symbols of the individual subcarriers considered. To minimize intersymbol interferences, each OFDM block is preceded by a guard interval TG in the time domain which causes an extension of the discrete-time OFDM signal in the interval [−TG, 0]; compare “Mitteilungen der TU-Braunschweig, Mobilfunktechnik für Multimedia-Anwendungen”, figure 7. The inserted guard interval TG advantageously corresponds to the maximum delay difference occurring between the individual propagation paths occurring during the radio transmission. By removing the added guard interval TG at the receiver end, a disturbance of the ith OFDM block by, for example, the adjacent OFDM signal in time at time i−1 is avoided, so that the transmit signal is received in interval [0, T′] over all indirect paths and the orthogonality between the subcarriers is retained to its full extent in the receiver. In the case of a large number of subcarriers, for example n=256 subcarriers, and correspondingly long symbol periods T=T′+TG, the period TG is small compared with T so that the insertion of the guard interval effectively does not significantly impair the bandwidth and only a small overhead is produced. After the transmit signal received at the input of the receiver is sampled in the baseband by an A/D converter, and after the useful interval has been extracted, i.e. after the guard interval TG has been eliminated, the received transmit signal is transformed into the frequency domain with the aid of a discrete Fourier transform (DFT); i.e., the received modulation symbols or, respectively, the received receive symbols are determined. From the receive symbols determined, the corresponding receive code words are generated via a suitable demodulation method, and from these the received serial data stream is formed by parallel/serial conversion. Avoiding intersymbol interference in OFDM transmission methods considerably reduces the computing effort in the respective receiver as a result of which the OFDM transmission technology is used, for example, for the terrestrial transmission of digital television signals; for example, the transmission of broadband data streams with a transmission rate of 34 Mbit/s per radio channel.
To transmit the serial data stream to be transmitted with the aid of the OFDM transmission method, absolute or, respectively, differential modulation methods and corresponding coherent or, respectively, incoherent demodulation methods are used. Although the orthogonality of the subcarriers is retained in its full extent by using the OFDM transmission method when transmitting the transmit signal formed via the “radio channel” transmission medium, both the phase and the amplitude of the transmitted discrete-frequency and frequency-selective transmit signals are changed by the transmission characteristics of the radio channel. The influence of the radio channel on amplitude and phase takes place subcarrier-specifically on the individual subcarriers which in each case have a very narrow bandwidth. In addition, noise signals are additively superimposed on the transmitted useful signal. When coherent demodulation methods are used, a channel estimation is required which depends on considerable technical and economic expenditure for its implementation depending on the quality requirements and which also reduces the performance of the transmission system. Advantageously, differential modulation methods and corresponding incoherent demodulation methods are used in which any elaborate radio channel estimation can be dispensed with. In the case of differential modulation methods, the information to be transmitted is not transmitted directly by selection of the modulation symbols or the discrete-frequency transmit symbols but by changing the discrete-frequency transmit symbols, which are adjacent in time, on the same subcarrier. Examples of differential modulation methods are the 64-level 64-DPSK (differential phase shift keying) and the 64-DAPSK (differential amplitude and phase shift keying) methods. In the 64-DAPSK, both the amplitude and simultaneously the phase are differentially modulated.
In the case of large delay differences between the individual signal paths, i.e. in the case of strong multipath propagation, different transmission-channel-related attenuations may occur between the individual received subcarriers with attenuation differences of up to 20 dB and more. The received subcarriers having high attenuation values or, respectively, the subcarriers having low S/N values (also called the signal power/noise power ratio) have a very large symbol error rate as a result of which the total bit error rate rises considerably over all subcarriers. In the case of subcarriers modulated with the aid of coherent modulation methods, it is already known to correct the attenuation losses caused by the frequency-selective transmission characteristics of the transmission medium (also called the transfer function H(f)) with the aid of the inverse transfer function (also called 1/H(f)) at the receiving end. The frequency-selective attenuation losses are then determined, for example, by evaluating reference pilot tones transmitted and in each case are allocated to certain subcarriers. This method for equalizing the transmission channel at the receiving end, however, causes a great increase in noise in the subcarriers with low S/N values. The bit error rate caused by the increase in noise in subcarriers with low S/N values cannot even be improved by introducing channel coding so that the total transmission channel capacity of the frequency-selective transmission medium, which is possible over all subcarriers, is not achieved in spite of equalization of the transmission channel at the receiving end.
In known methods for improving the transmission quality in multicarrier systems as are known, for example, from the document “Comparison between adaptive OFDM and single carrier modulation with frequency domain equalization”, A. Czylwik, IEEE Vehicular Technology Conference, USA, New York, vol. Conf. 47, 1997, pp. 865–869, XP000736731, ISBN: 0-77803-3660-7, the transfer function of the channel is estimated via information already transmitted. It is assumed here that the characteristics of the radio channel change only slowly in time. The estimated transfer function is transmitted back to the transmitter from the receiving station via signaling stations.
In a multicarrier method according to U.S. Pat. No. 5,673,290, transmission parameters of a communication line are measured. The modulation method of each carrier is then adapted to the measured parameters.
The present invention is thus, directed toward achieving maximum utilization of the available transmission resources of the transmission medium during the transmission of information via a transmission medium having frequency-selective transmission characteristics. In particular, it is intended to achieve maximum utilization of the transmission resources of all multipath components or subcarriers when using a multicarrier method.